As the world's need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are able to carry tens, hundreds, and even thousands of gigabits per second across substantial distances.
A conventional optical fiber is essentially an optical waveguide having an inner core and an outer cladding, the cladding having a lower index of refraction than the core. Because of the difference in refractive indices, the optical fiber is capable of confining light that is axially introduced into the core and transmitting that light over a substantial distance. Because they are able to guide light due to total internal reflection principles, conventional optical fibers are sometimes referred to as index-guiding fibers. Conventional optical fibers have a solid cross-section and are made of fused silica, with the core region and the cladding region having different levels of dopants (introduced impurities) to result in the different indices of refraction. The cladding is usually doped to have a refractive index that ranges from 0.1% (single mode fibers) to 2% (multi-mode fibers) less than the refractive index of the core, which itself usually has a nominal refractive index of 1.47.
As known in the art, single-mode fiber is preferred over multi-mode fiber for high-capacity, long-distance optical communications. Single-mode fiber prevents electromagnetic waves from traveling down in the fiber in anything but a single, tightly held mode near its center axis. This is in contrast to multi-mode fiber, in which incident electromagnetic waves may travel down the fiber over several paths of differing distances. Accordingly, single-mode fiber allows for reduced group delay, and thereby allows optical signals to better keep their shape as they travel down the fiber. As described in Dutton, Understanding Optical Communications (Prentice Hall 1998), which is incorporated by reference herein, at p. 45, single-mode fibers may be created by (i) making the core region thin enough, (ii) making the refractive index difference between the core and the cladding small enough, or (iii) using a longer wavelength. Conventional single-mode fibers have a core diameter of about 9 μm and a cladding diameter of about 125 μm, and are single-mode down to a cutoff wavelength of about 1100 nm, below which they become multi-mode.
FIG. 1 shows a conventional optical fiber communications link 100 comprising a source transmitter 102, a destination receiver 104, and a plurality of regeneration spans 106 therebetween. Optical fiber communications link 100 represents a long-haul, high-capacity optical communications link that would run, for example, between the cities of San Francisco and New York, or between the cities of Los Angeles and Las Vegas. Each regeneration span 106 comprises a plurality of amplifier spans 108 and a regenerator 110, and has a length LRG representing the distance between regenerators 110. Each amplifier span 108 comprises a fiber span 112 and an amplifier station 114 that houses a dispersion-compensating fiber loop 116 and an optical amplifier 118, with a length LA representing the length of the fiber span 112. Within each regeneration span 106 the signal from source transmitter 102 remains all-optical, while at each regenerator 110 the optical signal must be regenerated, i.e. converted into digital electrical signals and then reconverted back into a “clean” optical signal for continued transmission down the optical fiber communications link 100. Generally speaking, each regenerator 110 essentially comprises a combination of an optical receiver similar to the destination receiver 104 and an optical transmitter similar to the source transmitter 102, along with appropriate electrical circuitry therebetween.
In practical implementations, the fiber spans 112 are placed in buried conduit along rights-of-way owned or leased by communications companies, and run between consecutive amplifier stations 114 and regenerators 110 as shown in FIG. 1. As known in the art, each regenerator 110 may be coupled to equipment for adding, removing, or routing data to other communications links, usually when the digital data is in electrical form. Likewise, each amplifier station 114 may include equipment for optically adding or dropping channels at different wavelengths for coupling to other optical fiber communications links. Accordingly, the optical fiber communications link 100 of FIG. 1 generally represents one of several paths between an information source (e.g., in San Francisco) and an information destination (e.g., in New York) within a communications network. However, regardless of the overall communications network in which the optical fiber communications link 100 may be contained, it is important to note that a given information signal originating at the source transmitter 102, e.g., an information signal fi(t) on an optical carrier at wavelength λi, must generally be regenerated several times prior to arrival at destination receiver 104, and also must generally be optically amplified several times between regenerators.
Amplifier stations 114 and regenerator stations 110 represent substantial costs in installing and maintaining optical fiber communications link 100. Each amplifier station 114 requires housing in a manhole or other type of communications relay shelter, and requires a consistently maintained environment that includes a reliable electrical power supply for powering the optical amplifiers 118. Furthermore, each regenerator 110 comprises an extensive amount of high-cost optical, electro-optical, and electronic equipment that requires an even more stable and consistently-maintained environment, such as that provided by a telephone central office or other central telecommunications facility. In addition to the required costs, each amplifier station 114 and regenerator station 110 adds another possible point of failure for the optical communications link 100, either through a electrical/optical component failure or a power failure. Accordingly, in the design of optical fiber communications link 100, it is desirable to require as few amplifier stations 114 and regenerator stations 110 as possible between the source transmitter 102 and the destination receiver 104.
Also shown in FIG. 1 is a cross-section of a conventional optical fiber 120 used in each fiber span 112, the optical fiber 120 comprising a solid core region 122 surrounded by a solid cladding region 124. Conventional optical fibers suffer from several adverse effects that reduce the efficiency of information transfer and the practical distance over which information may be carried by the light. The two primary adverse effects are (a) attenuation, which is a reduction in signal magnitude as it travels down the fiber, and (b) dispersion, which is a loss of signal shape as different component wavelengths travel down the fiber at different speeds. These two adverse effects often overlap in their unfavorable impacts on optical communication system design. In general, however, attenuation effects serve to (i) reduce the range of wavelengths λ that may be used to carry information down a fiber, and (ii) reduce the required spacing between optical amplifiers for those wavelengths that are usable. Also, generally speaking, dispersion effects serve to (i) reduce the rate at which a light beam at a given wavelength λi may be modulated with information, and (ii) reduce the required spacing between regenerators. As described in Dutton, supra at p.398, attenuation and dispersion are the two critical factors in fiber optic communication system design. Other critical factors include the cost of components and the cost of putting them together.
As dictated by the attenuation and dispersion limitations of conventional optical fibers, today there is a large installed base of optical fiber communications links in which the maximum distance between optical amplifiers is about 100 km, and wherein the maximum distance between regenerators is about 500 km. It would be desirable to provide an optical fiber communications link in which the distances between optical amplifiers and the distances between regenerators is substantially increased, for reducing overall system cost and increasing overall system reliability. Alternatively, it would be desirable to provide an optical fiber communications link which, if substituted into the above installed base of amplifier stations and regenerators, would have substantially increased data throughput using these amplifier and regenerator distances. As a further alternative, it would be desirable to provide an optical fiber communications link which, while maintaining current data rates, would have substantially reduced system hardware requirements in terms of regenerator spacing, optical amplifier spacing, and/or dispersion-compensating fiber lengths, thereby substantially decreasing system construction and maintenance costs and improving system reliability.
Also as dictated by the attenuation and dispersion limitations of conventional optical fibers, conventional long-haul optical fiber links operate in a narrow band of wavelengths between 1530-1570 nm due to the fact that conventional Erbium-Doped Fiber Amplifiers (EDFAs) operate in this range, together with the fact that conventional single-mode fibers have a smaller attenuation (about 0.20 dB/km) in this range. Because it is desirable to carry as much information as possible in this 1530-1570 nm wavelength band, as many channels of information as possible are “squeezed” into this wavelength band using dense wavelength-division multiplexing (DWDM) methods.
DWDM systems modulate successive channels of information fi(t) onto successive optical carriers at wavelengths λi, which are then multiplexed onto a single fiber. In typical practical systems today, the bandwidth of each signal fi(t) may be about 10 GHz, while a typical separation between wavelengths λi may be about 1.6 nm. Although decreases in channel separation (e.g., down to 0.8 nm or smaller) can increase the overall capacity of the optical fiber, these decreases require more expensive, higher-quality optical filtering devices to separate the densely-packed channels from each other, as well as more expensive optical sources having precisely located center wavelengths and narrow wavelength spreads. Also, since the modulation process itself induces a wavelength spread for each channel (about 0.16 nm for 10 GHz modulation), the narrowness of the 1530-1570 nm wavelength band places an upper limit on the amount of information that can be carried even if the more expensive optical filtering devices and optical sources are used. Accordingly, it would be desirable to provide a long-haul optical fiber communications system in which the fiber spans operate at wavelengths additional to those at 1530-1570 nm, thereby providing for (i) increased overall information throughput, and/or (ii) wider channel separations that, in turn, permit the use of lower-cost optical filtering devices and optical sources.
FIG. 2 shows an attenuation plot 200 for a conventional silica glass optical fiber versus wavelength, for wavelengths between 800 nm and 1600 nm (see Dutton, supra at p. 40). Attenuation (α) is commonly expressed in dB/km and, as shown in FIG. 2, varies greatly with wavelength for conventional optical fibers. As described further in Hecht, Understanding Fiber Optics, Prentice-Hall (1999), which is incorporated by reference herein, at pp. 82-91, attenuation is primarily caused by absorption of the light by the silica glass and impurities therein, as well as by scattering of the light by atoms within the optical fiber. Generally speaking, attenuation limitations in the design of a fiber optic communications link are dictated by a transmitter power Pt (in dBm), a minimum receiver sensitivity Pr (in dBm), the fiber attenuation α (in dB/km), and the fiber span length LA (in km) through the relationship of Equation (1):Pr>=Pt−αLA   {1}
As known in the art, the receiver sensitivity Pr will vary inversely with modulation rate, with exemplary conventional values being Pr=−25 dBm, −20 dBm, and −15 dBm at 2.5 Gb/s, 10 Gb/s, 40 Gb/s, respectively. Accordingly, using α=0.20 dB/km for the DWDM wavelength range between 1530-1570 km, conventional maximum fiber span lengths LA between amplifiers would be about 125 km, 100 km, and 75 km at 2.5 Gb/s, 10 Gb/s, and 40 Gb/s, respectively. It would be desirable to provide an optical fiber communications link using an optical fiber having a reduced attenuation a such that distances between optical amplifiers may be increased for a given data rate, and/or such that data rates may be increased for a given spacing between optical amplifiers.
FIG. 3 shows a dispersion characteristic 300 for a conventional single-mode silica glass optical fiber versus wavelength, for wavelengths between 800 nm and 1600 nm. See Jopson, B., “Chromatic Dispersion Compensation and Measurement,” Optical Fiber Communication Conference 2000 Proceedings, Baltimore, Md., TuC, pp. 1-28, (2000), which is incorporated by reference herein. Dispersion (D) is commonly expressed in ps/(nm-km) and, as shown in FIG. 3, varies with wavelength. As described further in Hecht, supra at 72-76, dispersion in single-mode fibers is primarily chromatic dispersion, i.e., pulse spreading that arises from differences in the speed that light of different wavelengths travels through materials. Chromatic dispersion, in turn, comprises the components of (i) material dispersion caused by variation of a material's refractive index with wavelength, and (ii) waveguide dispersion caused by the different speeds that light travels in the core and cladding of single-mode fibers. Generally speaking, it is most desirable for a section of optical fiber to have a very low dispersion value (positive or negative) across the wavelengths of operation. However, as described in Hecht, supra at 100, the dispersion value should not be zero because a nonlinear effect referred to as four-wave mixing causes serious problems in WDM systems operating at zero-dispersion wavelengths.
Generally speaking, practical dispersion limitations in the design of an optical fiber communications link may estimated using any of a plurality of thumbnail design guidelines available in the literature. One such guideline is given in Elrefaie, A., Wagner, R., Atlas, D. and Daut, D., “Chromatic Dispersion Limitations in Coherent Lightwave Transmission Systems,” Journal of Lightwave Technology, Vol. 6, No. 5, pp. 704-709 (May 1988), which is incorporated by reference herein. According to this guideline, for an exemplary transmitter power Pt=0 dBm and receiver sensitivity Pr=−20 dBm, the length LRG between regenerators may be estimated for a given modulation rate B (in Gbps) and dispersion D (in ps/nm-km) according to Eq. (2):B2LRGD<(10 Gbps)2(50 km)(20 ps/nm-km)   {2}
Accordingly, using D=20 ps/(nm-km) for the DWDM wavelength range between 1530-1570 km, conventional distances LRG between regenerators would be about 800 km, 50 km, and 3.1 km at 2.5 Gb/s, 10 Gb/s, and 40 Gb/s, respectively.
In practice, as described in Hecht, supra at 72-76 and 96-97, the distance LRG between regenerators may be increased by (i) using dispersion shifted fibers in the fiber spans 112, and/or (ii) using dispersion compensating fiber loops 116 at regular intervals. Dispersion-shifted fibers have a refractive index profile adjusted such that a negative waveguide dispersion characteristic is intensified to compensate for a positive material dispersion characteristic. Because the negative waveguide dispersion characteristic is relatively flat, the net effect is to move the dispersion characteristic 300 of FIG. 3 to the right by fixed amount, with the zero-crossing moved to 1550 rim in zero-dispersion-shifted fiber or, for example, to about 1450 nm in nonzero-dispersion-shifted fiber. A portion of a dispersion characteristic of a zero-dispersion-shifted fiber is shown as plot 302 in FIG. 3, while a portion of a nonzero-dispersion-shifted fiber is shown as plot 304 in FIG. 3.
In contrast, dispersion compensating fibers are specially designed to have a large negative dispersion characteristic (e.g., −200 ps/nm-km) and are placed in series with the fiber spans at regular intervals. Dispersion-compensating fiber loops 116 are shown in FIG. 1 and, although shown in front of each optical amplifier 118, may alternatively be placed after each optical amplifier 118. Thus, for example, for every 10 km of fiber span 112 having a dispersion of +20 ps/km-nm, there would need to be 1 km of dispersion-compensating fiber having a dispersion of −200 ps/km-nm. As with dispersion-shifted fibers, the net effect of the dispersion-compensating fiber loops 116 is to move the dispersion characteristic 300 of FIG. 3 to the right by fixed amount. Disadvantageously, dispersion-compensating fibers introduce substantial attenuation into the fiber link 100, having typical attenuations of 0.5 dB/km.
More importantly, the use of dispersion-shifting or dispersion-compensating fibers does not alleviate substantial limitations on the range of operable wavelengths of the optical fiber link 100, because the dispersion magnitudes are shifted to smaller values only for a limited range of wavelengths. Disadvantageously, according to these methods, material dispersion is not itself reduced, but is merely compensated for by laterally shifting the dispersion characteristic 300 such that it is smaller in certain wavelengths of operation. The usable range of wavelengths over which the dispersion remains sufficiently low is still limited. In current practical implementations, these limitations are not problematic because of overriding attenuation limitations, i.e., the operable wavelength ranges are limited anyway by attenuation factors (e.g., to 1530-1570 nm for DWDM systems). However, in systems where a wider band of operable wavelengths is desired, the dispersion limitations caused by the steep slope of the dispersion characteristics 300 and 302 may become the limiting factors. Therefore, it would be desirable to provide an optical fiber having a low dispersion characteristic that also has a smaller slope when plotted versus wavelength. With such a dispersion characteristic, the optical fiber will have low but non-zero dispersion across a larger range of wavelengths, thereby expanding the range of operable wavelengths of the optical fiber. Moreover, it would be desirable to provide an optical fiber having a low dispersion characteristic such that the periodically-spaced dispersion-compensating fibers are not required or, if required, are of a lesser required length.
Accordingly, it would be desirable to provide an optical fiber communications link having increased spacing between successive optical amplifiers for reducing system cost and increasing system reliability.
It would be further desirable to provide an optical fiber communications link having increased spacing between successive regenerators for reducing system cost and increasing system reliability.
It would be still further desirable to provide an optical fiber communications link incorporating an optical fiber having reduced attenuation and a dispersion characteristic exhibiting smaller dispersion values across a larger range of wavelengths, and a smaller slope when plotted versus wavelength.
It would be even further desirable to provide an optical fiber communications link requiring fewer dispersion compensating fiber loops.
It would be still further desirable to provide an optical fiber communications link that is operable over a wider range of optical carrier wavelengths, such that overall WDM system bandwidth is increased and/or the cost of WDM optical filtering and optical source components is reduced.